In recent years, a method for manufacturing a tire has been proposed, in which a green tire is built by winding and laminating a rubber strip on a rigid core or a tire building drum, in order to realize a smaller production unit which does not need preparation of tire component members such as bead filler, belt and tread prior to building a tire so as to reduce process manpower and time, as well as to eliminate production equipments for such tire component members, thereby saving space for such equipments.
In this method, however, when the green tire is built by winding and laminating a rubber strip on the rigid core or the tire building drum, a stepped portion occurs between one part of the rubber strip and another part of the rubber strip overlapped thereon. During vulcanization of the green tire, the stepped portion formed from a tire crown section to a bead section cannot be sufficiently molded, so that bares and flow cracks may be present in the product tire, which cause problems of deteriorating the durability and an appearance of the product tire.
Moreover, air retained at the stepped portion in the tire does not escape but still remains during the vulcanization. Air enclosed between a surface of the tire at the stepped portion and a tire vulcanizing mold also remains in the tire as it is incorporated by a rubber flow during the vulcanization. As a result, there is a problem in using the product tire that, as stress concentrates at the air-remaining portion, the air-remaining portion is likely to have cracks as compared with the rest of the tire so that good the durability cannot be realized.
Further, a contaminant such as a mold-releasing agent easily deposits on the stepped portion formed at the sidewall. Thus, when the tire is expanded by the vulcanization, the contaminant pinched between the surface of the rubber and the mold is pressed into the rubber to make an contaminants-embedded portion on the surface of the product tire. This may also result in occurrences of the cracks, so that good the durability cannot be achieved.
In contrast to the above-mentioned method, there has been known a method for building a tire in which each tire component member such as bead filler, belt and tread is separately formed prior to building the green tire, and then these tire component members are bonded one another on the drum to build a desired green tire. For the production of the tire component member used in this method, a method has been also proposed in which the tire component member is built by winding and laminating a rubber strip on a tire component member forming drum.
When the tire component member such as bead filler is laminated by winding the rubber strip on the tire component member forming drum according to this proposal, a stepped portion is, however, made between one part of the rubber strip and another part of the rubber strip overlapped thereon, so that, as mentioned in the above, there is a problem of bare, remaining air and embedding of the contaminant during the tire vulcanization, which may cause a crack.
With reference to FIGS. 10-13, what has been described above is more specifically exemplified. FIG. 10 is a sectional view showing a green tire built according to the conventional production method. In the building of the green tire 90, an inner liner 91, a carcass layer 92, beads 93A, 93B, a sidewall 94 and belt layer 95A, 95B are formed, and subsequently a rubber strip 97 is helically wound and laminated to form tread 96.
The rubber strip 97 for forming the tread 96 generally has a flat rectangle cross section as shown in FIG. 1a. FIGS. 12a and 12b are cross-sectional views showing details of portions a and b of FIG. 10, respectively. In the building of the green tire, the rubber strip 97 having rectangle cross section are wound and laminated in such a manner that the laminated rubber strips are inclined and stepwise, a space remains between the sidewall 94 which has been already bonded and built and the stepped portion of the rubber strip 97 of the tread 96 as shown in FIG. 12a. The space retains air in the product tire and causes a decrease in the durability due to a stress concentration. The outer surface of the tread 96 is also formed like a saw blade as shown in FIG. 12b by means of the stepped portion of the rubber strip 97, which cause the above-mentioned bares, flow cracks and the like.
In order to solve the problems, in Japanese Patent Application Opened No. 2000-254980, it is proposed to use a rubber strip 99 having a cross section as shown in FIG. 11b. The rubber strip 99 is provided with a maximum thickness portion 99a and an edge rubber portion 99b having a rectangle cross section whose thickness is smaller than that of the maximum thickness portion 99.
FIGS. 13a and 13b are cross-sectional views showing details of parts of tread 96 of a green tire 90 corresponding to parts a and b of FIG. 10, respectively, which is built by winding and laminating the rubber strip 99 having the edge rubber portion 99b instead of rubber strip 97 shown in FIG. 10. In the rubber strip 99, the edge rubber portion 99b also has a rectangle cross section, so that, when the rubber strip 99 is inclined, wound and laminated, a space is formed and remains at a stepped portion near the edge portion 99b having a rectangle cross section and, therefore, an occurrence of residual air is inevitable. The outer surface of the tread 96 cannot be sufficiently smoothened since the edge rubber portion 99b having a rectangle cross section is stepwisely laminated as shown in FIG. 13b. Thus, it does not solve the problem of occurrences of bares, flow cracks and embedded contaminants due to the saw blade-like surface.
The present invention is directed to solve these problems which the conventional art has, and the object of the present invention is to provide a rubber strip for improving the durability of a product tire by reducing residual air and an amount of a step in a stepped portion formed with one part of the rubber strip and another part of the rubber strip overlapped thereon prior to vulcanization so as to suppress problems in the product tire which are likely caused by occurrences of bare and a flow cracks, residual air and embedded contaminants, and a method for manufacturing a tire and a tire component member using the rubber strip, and an apparatus for the method.